Cache color space instances

Details

Description

I have a PDF from a customer which contains a lot of calls of SetNonStrokingColorSpace and SetStrokingColorSpace. Each time, an embedded color profile resource is loaded via ICC_Profile.getInstance(InputStream). I have attempted to cache the result in PDResources.java as shown in the attached patch. For this particular PDF, this change improves the performance of PDFToImage from 27 seconds down to 5 seconds (the PDF has two pages). I cannot share the customer PDF, so I have attempted to find a similar free one. Unfortunately, in my test suite, I did not find anything with a comparable improvement. The best example I found is in the attached PDF. There the improvement is from 4.9 seconds without caching to 4.1 with caching.